UPDATE: S2 Games has confirmed that only passwords were stolen when a piece of third-party software was breached. No credit card or billing information is stored by S2 and was not compromised. The original story follows.
Heroes of Newerth's password security has been breached, and developer S2 Games is advising any players who used the same passwords across multiple services to change them. S2 Games noticed the breach Sunday afternoon and posted a statement to the game's official forums that evening.
S2 Games took all of the multiplayer online battle arena's services offline this morning to fix the issues, and turned them back online as of this afternoon. Users will need to create a new password when logging in the first time since the breach.